The present invention relates to an economical composite explosive with good bulk strength. More particularly, in one aspect the invention relates to a combination of an emulsion containing a water soluble fuel and an aqueous phase which contains no dissolved oxidizer salts with particulate ammonium nitrate or ANFO to achieve a dense composite explosive. In another aspect the invention utilizes an emulsion having water soluble fuels and an unsaturated aqueous solution of dissolved oxidizer salts as the discontinuous phase of the emulsion component.
It has been common practice in the explosive art to utilize oxidizer solutions which contain the maximum amount of oxidizer possible, i.e., the highest possible oxidizer to water ratio. This served a dual purpose. First it provided for a maximum energy on a weight basis. This is because water essentially acts like an energy sink. Second it was believed to be necessary to provide the greatest sensitivity to initiation.
This point of view has been carried over the blends of emulsions with solid AN or ANFO since it represented the common wisdom. However, maximizing oxidizer causes several difficulties. First it adds cost to the final explosive in terms of raw materials. Second it requires the explosive manufacturer to have facilities that are capable of handling oxidizer solutions at relatively high temperatures. This of course creates safety as well as economic and logistic difficulties. This second problem can be partially rectified by the use of calcium nitrate or sodium nitrate to lower the crystal point of the oxidizer. These ingredients of course represent additional oxidizer salts and correspondingly increased cost.
Contrary to the current practice, it has been found that the use of oxidizer salts in the emulsion portions of the blend can be minimized and still result in a product with a high energy and good sensitivity to initiation. This allows the use of low a crystal point and essentially oxidizer free aqueous solutions to be used as the internal phase of the emulsion protion of the emulsion/solid oxidizer product.
The explosive industry has continuously sought inexpensive and easy to handle explosive mixtures. Mixtures of ammonium nitrate and diesel fuel have been used for many years in the explosive industry to produce ANFO. ANFO currently is typically a blend of ammonium nitrate ("AN") in prill form mixed with diesel fuel oil ("FO") in a ratio of about 94% AN to 6% FO. ANFO is inexpensive and is widely used in various kinds of blasting, but its relatively low bulk density (about 0.8 gm/cc) limits the amounts of useful energy that can be obtained in the borehole. Further, ANFO suffers the disadvantage that it becomes desensitized by water, precluding its use in water filled boreholes.
The industry has made various attempts to increase the bulk strength of ANFO to thereby provide more energy per unit of volume. One method attempted was to increase the density of ammonium nitrate by using ground ammonium nitrate rather than prills. Another attempt was use of a high density additive fuel to the ANFO, such as ferrophosphorus. U.S. Pat. No. 3,764,421 describes an attempt to increase the density of ANFO by addition of a controlled amount of water and aging the mixture for a number of days to soften the ANFO, followed by mixing of the ANFO after aging to break it down into finely-divided solids.
Australian Pat. No. 281,537 describes explosives using ammonium nitrate prills mixed with an emulsion formed from oil, water and a surface active agent or emulsifier. This was an attempt to improve the sensitivity of ANFO by adding a small amount of water to the ammonium nitrate and distributing with oil in the form of an emulsion over the ammonium nitrate. This was proposed in order to better distribute the water and oil over the prill and for the intended purpose the emulsion did not need to be stable.
To provide a waterproof product the explosive industry developed water-in-oil emulsions, such as disclosed in U.S. Pat. No. 3,447,978. The water-in-oil emulsion formed contained a saturated aqueous solution of oxidizing salts which form the discontinuous phase of the emulsion. The emulsion contained an occluded gas component to lower the density of the emulsion to render it detonable.
Solid oxidizer salts or ANFO have also been combined with emulsions with saturated aqueous phases containing dissolved oxidizer salt. U.S. Pat. No. 3,161,551 discloses a water-in-oil emulsion which was combined with particulate ammonium nitrate so as to fill all the spaces between the solid particles of ammonium nitrate. U.S. Pat. No. 4,111,727 disclosed a composition formed by mixing 10 to 40% of a water-in-oil emulsion containing an oxidizer salt dissolved in the aqueous phase together with 60 to 90% of a solid oxidizer, such as ammonium nitrate, in such a manner that sufficient air is left in the interstitial spaces of the solid oxidizer to render the mixture detonable.
Water-in-oil emulsions containing an oxidizer salt dissolved in the aqueous phase and sensitized by occluded gas, such as glass microballoons, have had mixed into them solid oxidizer salts. For example, U.S. Pat. No. 4,181,546 discloses a waterproof composition containing 40 to 60% by weight of a solid oxidizer salt to the extent to 60 to 40% of a water-in-oil emulsion sensitized by microballoons. See also U.S. Pat. No. 4,509,994.
Mixtures of the solid oxidizer or ANFO with emulsions have varying degrees of stability. Various attempts have been made to improve the stability of explosive compositions utilizing particulate oxidizer salt. In U.S. Pat. No. 4,555,278 the stability of a blend of nitrate particles in a water-in-oil emulsion is thought to be improved by controlling the salt size of the dispersed aqueous phase in the emulsion so as to decrease the chemical driving force between the water and the solid oxidizer. In another approach, in U.S. Pat. No. 4,585,495 the stability of a mixture utilizing solid oxidizer is thought to be improved by utilizing an aqueous slurry having a blend of fuel oil and saturated oxidizer slurries of water-retemptive. In yet another approach, U.S. Pat. No. 4,294,633 discloses a composite mixture in which solid oxidizer particles are combined with a liquid slurry which does not contain water but rather utilizes a low molecular weight polar organic liquid such that the slurry is one that will not appreciably dissolve or soften the solid oxidizer salts.
The industry has constantly sought for improved blasting compositions to provide the most economical explosive having acceptable power output together with desirable handling characteristics. Heretofore the art, when preparing oxidizer solutions for use as the aqueous phase of water-in-oil emulsion, utilized saturated oxidizer solutions. The present invention provides for a very economical product which is easy to manufacture and utilize, while avoiding many of the less desirable characteristics of emulsion having saturated oxidizer solutions.